Radio receiving circuits



June 12, 1934. P. H. GREELEY 1,9.62,557

RADIO RECEIVING CIRCUITS Filed July 29. 1930 2 Sheets-Sheet l v v v v v v v' June 12, 1934. p H GREELEY 1,962,557

RADIO RECEIVING CIRCUITS Filed Ju1y`29, 1930 z Sheets-shem 2 ga "jr-74% PZ M2 M2 52 Phil/lp H. ree/ey (IlA Patented June 12, 1934 UNITED 4 srA'rEs PATENT OFFICE 5 Claims.

My nvention relates to radio circuit improvelnents particularly applicable to the design of radio rceiving sets capable of receiving a wide range of transmitted frequencies and adapted for operation from alternating current as well as batteries, as may be desired. It is generally agreed that alternating current operation introduces problems not present in battery operation l and there is a general condition of combination between a receiving circuit and its power supply. If the problems of alternating current operation are satisfactorily solved, there is seldom any difficulty in arranging for battery operation, but the Converse is not usually true. v

It is also well understood that reception in different frequency ranges offers materially different problems and a receiver design should successfully meet varying conditions in the different frequency ranges to be capable of satise v factory results in all of them. Further, any recever intended for reception of telephone signals should provide satisfactory selectivity Characteristics without serious discrimination against 4necessary Voice and musical frequencies transmitted regardless of the radio carrier frequency received.

The objects of this invention therefore are:--

To provide a circuit design capable of giving high and uniforrn amplification with good selectivty and Without serious discrimination against any useful audible tone frequencies for use over a range of radio carrier frequencies:

To provide a convenient and advantageous form of a selective filter:

i To provide a circuit that is readily changed in character as well as frequency range:

To improve conditions with respect to alternating current operation of a receiver to reduce such operation difficulties to a minimum,

And, in general, to simplify such receiver control devices and changes as may be necessary to obtain satisfactory convenence and performance in the different frequency ranges to be received.

Referring to the drawings:

'Figure 1 is a schematic diagram of a receiving circuit and electric power supply embodying the features of my invention;

Figure 2 is a schematic diagram illustrating the design characteristic of my radio amplifying circuit;

Figure 3 is a schematic diagram illustrating the design of my filter circuit;

Figure 4 is a schematic diagram showing coil designs adapted to make a receiver most eflfective at differing frequency ranges;

Figure 5 shows schernatically an arrangement givng simplified control of a receiver;

Figure 6 is a diagram of a selective switching arrangement adapted to make desirable changes in a receiver with a minimum of difiiculty, and

Figure '7 is a schematic diagram for an alternative audio amplifier circuit.

In the drawings, A1-A1 may be taken as a line separating a first radio amplifier stage including a vacuum tube Tl and its input circuit from a second radio amplifier stage including a vacuum tube T2 and its input circuit, A2-A2 is a line separating the second amplifier stage from a third radio amplifier stage including a vacuum tube T3 and its input circuit, A3-A3 is a line separating the third amplifier stage from a detector stage including detector tube T4 or other rectifier and its input circuit, A4-A4 is a line separating the detector stage from a following three stage audio amplifier including vacuum tubes T5, TG and output tube T7, and below this three stage audio amplifier is shown a power supply circuit including a rectifier tube TS, a power transformer T9 and a filter and Voltage divider connected with the rectifier tube output.

Tubes Tl, TZ and T3 are illustrated as screen grid tetrodes of the type designed for alternating current operation having a filament, separate cathode, control grid, screen grid and plate or anode, though other amplifying devices in greater or less number may be substituted. Tubes Tfi, T5 and TG are illustrated as triodes of the type having a filament, separate cathode, grid and plate and tube T7 is illustrated as a power amplifrer tube having filament cathode, grid and plate, i

all of said tubes being of familiar types used for alternating current operation though other amplifying devices or detectors may be substituted in greater or less number. The rectfier tube TS is of the full wave filament type, though other devices may be substituted.

Where high radio frequency amplification is expected, it is common practice to shield carefully each amplfying stage and also by-pass currents carefully and use resistors or chokes to prevent common coupling in the circuits supplying current to the amplifying tubes. Such resistors and by-pass condensers are indicated on the drawings without any special description unless they must be considered as aifecting the novel features of this invention. In a receiving set of this general type, some shielding may be necessary or desirable and would naturally follow the separation lines between the various radio amplifying stages; it is best also to include each tube working at radio frequencies in the shield with its input circuit and also separately shield the base and socket of each screen grid amplifier tube or otherwise provide a set construction preventing undesirable coupling effects.

In the radio frequency stages, L1, L2, L3, and L4 are primary inductance coils, Pl, P2, PS, and Pil are primary coils induotively coupled to secondary tuning coils Sl, 52, S3, and S4 respectively. C1, CZO, C2, Co, C3, 040 and Ce are variable tuning condensers all of which may preferably be mounted on a single control shaft for simplified operation. Prirnary circuits may be coupied capacitively, as Well as inductively, to their respective secondaries through condenser Cll, C12, 013, and 014 o-r direct connections may be substituted for these condensers if inductive coupling only is desired. Grid returns for the several tubes may be made through resistors or cholzes R1, R2, R3 and R4 With connection to ground and negative side of the current supply where the tube cathodes are conneoted to a suit-- able positive potential, or other arrangements for proper operation of the tubes may be used.

For tuning circuits in a medium frequency range such as the broadcast band from 550 to 1500 kilocycles, tuning condensers having a maximum capacity of about 350 micro-microfarads are desirable, whereas for higher frequencies tuning condensers of smaller capacity are 'desirable because they permit a better ratio of tuning inductance to capacity to be used. Also, for tuning in the medium frequency range, it is common practice to provide four or five tuning circuits to obtain required selectvity Without resorting to the use of regeneration or very sharply tuned simple resonant circuits while for receiving the higher frequencies, fewer tuned circuits and the use of a regenerative detector has been found more satisfactory. Therefore, for reception in said medium frequency range, a three stage radio frequency amplier and detector circuit having five tuned circuits as indicated in stages A1, A2, A3 and A4 may be used, while for reception in the higher frequency ranges a single tuned radio frequency stage A3 and regenerative detector stage A4 may be employed without using stages Al and A2.

- For Changing the frequency range of a receiver, it is necessary to change the coils or otherwise switch in different tuning elements, the Changing of coils being most desirable from the standpoint of efficiency. In stages A3 and A4, interchangeable coils are indicated, each coil form and socket having five terminals lettered G, C, P, Fl and FZ similar to the markings on standard sockets for five prong tubes. Coils are Wound on suitable forms with end connections to terminals on the coil forms, set wiring being made to the sockets for the coil forms. Coil forms of various types with different socket arrangements may, of course, be used, though it is desirable to limit the number of contacts required on each coil to about five.

The circuit arrangement of stages A3 and A4 is adapted to permit different coils, different tuning capacities and materially different circuit arrangements to be used by simply Changing coils; also, the Steady currents or potentials required for operation of the tubes are applied without change whether or not the coils are in the circuits. Tuning condensers C3G and C3 or 040 and C4 may be used in parallel when link connection C--G on the coil forms is made, or C3 may be used alone when this link connection is omitted, or C3 and 030 may be used in series with coils designed for the higher frequencies. Condensers C3 and 030, or C4 and C40, in parallel should accurately match the capacities of condensers Cl, 020 and C2 when all are operated on a single shaft for simplified control, and small compensating condensers may be used in parallel with the different tuning condensers where required to put all tuning circuits in step.

When receiving in a medium frequency range as from 550 to 1500 kilocycles, the antenna Ant.) is switched to contact 1 and is preferably connected through a potentiometer Volume control device R7 to coil L1, the point of contact of R7 to L1 being variable if desired. Radio frequency signals are then selected and amplified in tubes Tl, T2, T3 and T4 and their coupling cireuits, being detected in tube T4 and then amplified at audio frequencies in tubes T5, TG and T7. Regeneration may be used on the detector by any suitable arrangement such as that employing a tickler coil P5 and a variable regeneraton control condenser C6, while for an nonregenerative detector, tickler coil P5 is omitted.

A particular featiu'e of my invention resides in the type of circuit employed for tuning the input of tubes Tl, T3 and T4 of Figure 1. Features of this general circuit may be better explained by reference to Figure 2, since it is necessary to take into consideration elements of design and electrical values employed to accomplish desired results to distinguish this arrangement from others previously described by myself and others.

Where a single tuning circuit is used in each amplifying stage, as is general practice, the secondary circuit of a transformer is usually the tuned circuit. The primary circuit of such a transformer is not tuned to any frequency within the tuning range of the secondary circuit and is most often designed to resonate because of its self inductance and such capacities as may be effective across it at some frequency higher than those within said secondary tuning range. In some cases, however, the primary circuit is made resonant to some frequency below those Within the secondary tuning range. Primary resonance points are not of great importance when the primary coil resistance is large relative to its inductance or where such resonant points or frequencies are relatively far from the frequencies within the secondary tuning range. Radio frequency choke coils often used for parallel feed of plate current are usually designed to have no appreciable resonance effects at the frequencies at which they are used.

In my circuit arrangement, I employ a primary circuit having an important resonant point below the lowest frequency within the secondary tuning range and another important resonant point higher than the highest frequency within the secondary tuning range. The circuit coupling tubes TZ and T3 of Figure 1 is shown more clearly to illustrate important features in the equivalent circuit of Figure 2. L3 is an efficient inductance coil having low resistance relative to its inductance such as may be made by winding a single layer solenoid, though L3 may be made small in size by using somewhat finer Wire than is preferable in a secondary tuning coil such as S3. L3 has some self capacity and there are tube element and other small capacities eifective across it, which capacities limit the esirable inductance of L3 to a value that Will place the resonant frequency of L3 With Cf, the fixed capacities ra f across it,rhighe'r than any frequencyl within the secondary tuning range. The parallel combination of L3 with Cf may be then be considered as a series impedance Z5 comprising a reactance :XS and resistance r5.

Other primary circuit elements comprise a primary coil P3 having some resistance r3P, a condenserC23, and a` mutual condenser C13 whose use is optional. Coupling between primary and secondary circuits is effected through mutual inductanoe M3 and mutual condenser C13, when used, though it is most convenient to speak of the coupling reactance 7'X3, however, effected. B'y summing up primary reactances and resistances in series, including :X5, 1'5, 7'X3 and the impedance Z10, the total primary impedance VZl comprising a reactance Xl and resistance rl is obtained. Similarlyl, the secondary reactances and resistances comprising 1'X3, Z20 and y'X4 may be considered in series to give the secondary impedance Z2 comprising a reactance jXZ. and resistance r2. Rp of Figure 2 is the A. C. plate resistance of amplifying tube TZ, Figure l, El is the amplified input Voltage from this tube or .,uEg1, and E2 is the Voltage available for input to tube T3. The ratio of E2/E1 is of particular interest in an Vamplifying circuit and may be written from the equivalent circuit of Figure 2 [E2 -X3X4 [Ei X2(B-D)-K1-j[X2(A-c)-K2] Where A general formula of this type is complicated but `X1/HX5 is of greatest interest since it may be controlled by circuit constants and design. The secondary resistance R2 is, of course, important, but the design of efficient Vsecondary tuning circuits is well understood.

In accordance with this invention, 1'X5 is made to act as an inductive reactance at frequencies within the secondary tuning range and iXl is also made to act as an inductive reactance over the same range, but Xl is made smaller than 9'X5v by the reactanceof cond'enser`C23 which is so chosen in value to make a'Xl zero at a frequency lower than any within the secondary tuning range. Within the secondary tuning range, then, the primary reactance :Xl is smaller than the effective output reactance at the amplifying tube. Reactance y'Xl is considerably smaller than a'X at the low end of the secondary tuning range but increases more rapidly with frequency and (iXl will approach but not exceed the value of 7`X5 at the high frequency 4end of the secondary tuning range. Therefore, the ratio Xl/X increases from a value considerably less than unity and will approa'eh but not exceed unity with increase vof frequency within the secondary tuning range.

The effect of ratio XI/X5 acting as described is to permit the `Voltage ratio E2/E1 to be greater at low frequencies than would b'e the case in a circuit having similar coupling and other constants but without vsatisfying the same primary circuit conditions. This gain in amplification at low frequencies is obtained without any sacrifice of gain at higher frequencies, as would be the case if the ratio X1/X5 were permitted to exceed unity at higher frequencies. Also, when using high impedance tube such as those of the screen grid type having a high amplification constant but high mpedance, the value of the coupling reactance may be considerably lower than is ordinarily considered necessary to obtain good actual amplification from such tubes. It is a real advantage to obtain satisfactory amplification Without the requirement of a very high reactance coupling primary and secondary circuits, since a high value of coupling reactance presents serious design difficulties if radio frequency transformers are to be efficient.

Another particular advantage of the circuit of Figure 2 is that there is less discrimination against audible tone frequencies at the lower radio frequencies received, where circuits of familiar types often are at fault. At high Vfrequencies, this circuit behaves essentially like a simple transformer coupled circuit having a relatively small primary and tuned secondary but at high frequencies the usual circuit resistance is sufficient to prevent serious side-band or audible tone frequency discrimination. When tuning toward the lower frequencies, however, the circuit approaches, to an increasing extent with decrease of frequency, the characteristics of coupled circuits resonant near the same frequency. This effect may be seen by con'sidering the reactance or 7` term of formula (1). At the tuning point, the 9' term is practically balanced out; that is, term KZ is practically bala-need by X2A. Off the tuning point, X2 will have some other value differing by a difference dX2. For a definite difference X2, the effect will be Smaller the Smaller the value of A, and I have made A"` decrease in value with decrease of frequency by controlling the ratio X1/X5 as explained above.

I have shown the coupling reactance between primary and secondary circuits with a combined inductive and capacitive reactance which may be in aidi'ng phase. The coupling condenser C13 may be omitted, but if used, the eifective coupling reactance may be made to vary less rapdly with frequency than is the case with a plain inductive reactance which varies with the first power of the frequency. When used, the combined coupling reactance may be made to vary about with the square root of the frequency. Capacitive coupling, even when the reactance is relatively small, appears to broaden the secondary resonance peak and may be used for this purpose to lessen side-band suppression and thereby improve 'audio tone quality. A slight arnount of resistance may be added to the coupling reactance y'X of Figure 2 to accomplish a similar purpose. It will be understood that any known means for obtaining a coupling reactance may be used in place of that shown without altering the features of this inventionr.

A distinguishing feature of this invention is that the reactances effective across the radio freduency line between tubes, as 7`X5, :XB and y'X4 of Figure 2, all have a nature similar to inductive reactahees and var'y with fre'quency.

`across only a portion of L1.

[pling the first tube to -P2 and 52 will therefore be considerable.

Although 7'X4 is a capacity, it is a variable tuning element and its actual value is changed when the circuit is tuned, such Changing having the effect of making the reactance 7'X4 vary directly with frequency instead of inversely with frequency as is the case with a fixed capacity. If desired, the input and output connections for the circuit network of Figure 2 may be interchanged with a general similarity of results obtainable from the circuit.

The antenna circuit of Figure 1 may be arranged to have Operating conditions similar to those explained with regard to Figure 2 for the inter-tube circuit net-work. In the inter-tube circuit, Cf is small and L3 preferably is made to have about the same inductive value as the secondary coil 83, and condenser C23 will have about the same or slightly greater capacity than the maximum capacity of the secondary tuning condenser, C3 or C3 and C30 in parallel. In the antenna circuit Cf may be much greater in value and L1 of Figure 1 corresponding to L3 of Figure 2 may be made smaller in inductive value so that the parallel combination of in- `ductance and capacity is resonant to a higher frequency than any within the secondary tuning range. Then C21, the primary circuit condenser corresponding to C23, will be made larger to make the primary circuit resonant to a fre- 'quency below any within the secondary tuning range. These conditions will hold when the antenna and ground connections are across the full coil L1, but L1 and C21 may be more like L3 and 023 if the antenna and ground connections are A potentiometer Volume control R7 should be of a value having as little effect on the primary circuit as possible. It will be understood that I do not limit myself to the use of this particular type of circuit couthe antenna circuit since any known arrangement may be substituted.

In order to improve the selectivity of a radio frequency amplifier without reducing the amplification of side bands necessary for best audio vquality, the circuit of stage A2 of Figure 1 illustrated in its essentials as the equivalent circuit of Figure 3 may be used. It is lmown that two tuned circuits resonant to approximately the same frequency, when loosely coupled together,

show a resonant curve that is nearly fiat for a limited range of frequencies with sharp discrimination against frequencies outside of this limited range. The distinguishing feature of my circuit illustrated in Figure 3 is that coils P2 and S2 are wound on the same form and may be in fact but a single coil tapped at or near its center. Under such conditions, the mutual inductance M2 between PZ and S2 will usually be rather large and the eifective coupling between If, however, PZ and 82 are wound in the same direction like a single continuous coil, M2 will be a negative mutual inductance, M2 being in eect added to PZ and 52 considered in series and subtracted from P2 and S2 considered in parallel.

In order to reduce the effective coupling reactance y'Xm, I employ a second mutual or common inductance reactance M1 which is positive instead of negative and makes M1 equal to, or

*somewhat smaller or greater than M2 by an saying space and simplifying the Vapparatus necessary for desired results. In the circuit of stage A2 of Figure 1, it is not necessary to use the same arrangement for supplying direct current to the plate of tube Tl with the direct current blocking condenser C12 since other methods of handling direct current plate or grid connections for the tube or tubes employed may be used Without materially Changing the radio frequency characteristics of the circuit. Likewise, if tube Tl is omitted, the antenna and ground may be coupled to the circuit of stage A2 in any suitable manner.

The circuit of Figure 3 may also be used in a filter circuit for rectified alternating current such as the output of rectifier tube TS. Such a filter circuit may comprise a tapped choke coil P20, 820, condensers C201 and C202, choke M10 and condenser C121 which correspond to the elements of Figure 3. By means of a tapped choke coil, the series inductive reactance of each part of the choke, P20 and S20 or P2 and S2 of Figure 3, is increased by the mutual inductive reactance M2, and less wire and core is required than is needed in two separate chokes of the same total inductance in series. For most effective filtering, the coupling reactance Xm should be made very low or zero at the output frequency of the rectifier tube which is 120 cycles in the case of the commonly used full wave rectifier used on a 60 cycle A. C. supply. The saving in Wire in the part of a filter circuit which must pass direct current is important in that there is less resistance and less Voltage loss for a given size of Wire used and the effectiveness of the filter.

As regards the circuits of stages A3 and A4 of Figure 1 which are shown adapted for interchangeable coils extending the frequency range of the tiuiing circuits, coils of different types changing the circuit action may be used as well as coils merely differing in size or inductive values. When Operating at frequencies different from the ranges for which stages A1 and A2 are designed, it is desirable, though not necessary, to restrict coil Changing' to two stages and eliminate additional stages. When stages A1 and A2 are eliminated, the antenna will be switched to point 2 or point 3 as may be desired. Point 2 provides aconnection through a series variable condenser C5 to coil socket terminal p while point 3 provides a direct connection to coil terminal Fl. If desired, the series variable condenser C5 may be provided with a switch shorting this condenser directly or through a larger capacity condenser, in which case, the switch point 3 may be ornitted. A switch operated by condenser C5 would, of course, be closed only at one setting of condenser C5, a simple cam mounted on the condenser shaft being suitable to operate such a switch.

One type of antenna coupling coil is shown in stage A3 and comprises a primary P3 and secondary S3. The link connection G-C may be closed or left open on the coil depending upon whether tuning condensers C3 and 030 are to be used in parallel or C3 is to be used alone. Also, the end of P3 may be connected to coil terminal Fl as shown or to coil terminal P if the series condenser C5 is to be brought into use. The detector tuning circuit of stage A4 is similar except that a tickler coil P5 for making tube Tfi regenerative is provided.

In order to reduce the effect of the relatively low input resistance of a grid leak detector on a tuning circuit connected to it, `I have used the coil connections shown in stage A4, where con- ``denser C40 is connected` across the full coil Sf and condenser C4 is connected` to a tap on coil 84. This arrangement makes C4 with S4 the main tuning circuit and keeps the detector input resistance from being eifective directly across this circuit.l

lIn Figure 4-A is shown an antenna coupled tuning coil for use in stage As of Figure 1, this col having a primary P3a c'onnecting` with the antenna through series condenser C5 and a secondary-S3 connected to be tuned by variable tuning condenser C3. v

A coil connected as a tuned impedance Sfiib is shown in Figure 4B. Regeneration is provided by a coil P5b. Tuned impedance coils may be used in both antenna stage A3 and detector stage A4 of Figure 1, though coil Pb for regeneration is not required in` stage A3.

For use at relatively high frequencies, coils of the type shown in Figure 4C have some advantages over the others shown. Coil S4c is used as a tuned secondary coil with Condensers C4 and C40 effective in series across them. Condensers in series decrease the eifective tuning capacity and the inductance of the secondary coils may be increased accordingly. With increased secondary tuning inductance, more eifective magnetic coupling between primaries Pfiic and S4c is readily provided, also, such capacity coupling as may exist between the primary coil wires and the C" ends of the secondary coils may aid the magnetic coupling. Although capacitive coupling is commonly employed in short wave receivers in preference to magnetic coupling, magnetic coupling apparently is better adapted for obtaining relatively noise free reception, par-- ticularly. Where alternating current operated tubes are employed. By careful coil designs of the types of Figure 4--C, I have been able to make the tuning controls for a radio frequency `amplifier stage and regenerative detector stage operate very closely in step, in step operation being, difficult to obtain with capacity coupled circuits.

The circuits of Figure 4-C also offer the possibility of eliminating separate primary coils by using capacity coupling to the C ends of the secondaries rather thanthe G or grid ends. The elimination of a separate tickler coil in the regenerative detector circuit in this manner is shown in Figure 4-D. Here the regenerative feed-back is made through the secondary S4d near or at the C end. It is preferable to make condensers C4 and C40 different in capacity, say C4 has a maximum of 125 micro-microfarads and 040 has a maximum of 250 mmf., such relativecapacities of C4 and 040 being used to provide an eifective capacity coupling for regeneration or primary input favorable to best operation of a receiver.

Besides making C4 and C40 of different maximum capacities, more uniform operation of the circuit over a frequency range covered by a tuning coil may be obtained by making the effective capacity coupling to a primary input or tickler circuit vary in a suitable manner, that is, by making the effective capacity coupling relatively small ati the high end of the frequency range and relatively great at the low frequency end. If condensers C4 and 040 are of the usual interleaving type, this effect may be obtained by making the plates of the condensers of differing Shape, C40 having plates giving approximately are designed for a relatively low frequency tuning range and a different coil and condenser suitable for a high frequency tuning range are required to make the circuit function as described in said higher frequency range. L4 and 024 need not be removed from the circuit since they do not have much effect at relatively high radio frequencies. L3e is shown not coupled inductively With secondary S3e and primary Pee which is preferable for best results.

It will be understood that the coil features and circuit conditions described herein are not limited to any one of the several known classes of receiving or transmitting circuits. It is well known that the super-heterodyne receiver has a number of advantages where a high powered receiver adapted for an extended tuning range is desired and the features of this invention may be adapted to this type of receiver to obtain uniform performance.

In order to make a receiving set of the class described operate well under different conditions and be simple and easy to use, several additional features are provided. When using morel than one stage of radio frequency amplification, a non-regenerative detector circuit arranged to be capable of handling relatively strong signals without serious distortion is desirable, while a more sensitive detector arrangement is preferable when only one stage of radio frequency ampliflcation is provided and the detector is regenerative. Considering the well known grid leak and condenser method of detection designed for strong signals and designed for sensitiveness to weak signals, the strong signal detector employs a relatively low grid leak resistance and higher plate Voltage is applied to the detector tube. Also, the grid of the strong signal detector may be preferably biased somewhat negative While the sensitive grid may be operated without bias.

By means of a simple switch I have provided for operation of a detector tube either as a sensitive or as a strong signal detector. R4 in Figure 1 is a relatively low grid leak resistance, R12 is a relatively high grid leak resistance, and R13 is a low resistance or direct connection. By means of a switch S5, the circuit through R13 may be opened and closed; at the open position, the grid leak resistance for tube T4 is high and at the closed position, the grid leak resistance for tube T4 is relatively low and also gives the grid of said tube a negative bias. The plate Voltage applied to tube Te is supplied from the same circuit as that supplying plate current to the first audio amplifier tube T5. This plate current supply has in it a series resistance R14 and a switch SG is provided for shorting this resistance. With switch SG open, the plate voltage applied to tubes T4 and T5 is lowered by the Voltage drop in resistance R14. and When switch SG is closed, the plate voltages are higher because there is no Voltage loss in R14. Switches 150 S5 and SG should be operated together, any simple double pole, single throw switch being suitable.

Although I have shown a grid leak tube detector, a similar switching arrangement may be provided for a grid bias detector. Also, the detector plate resistance and shorting switch may function on the detector tube alone and not on the audio amplifier tube T5 if desired. I prefer the audio amplifier and detector circuits shown because they make use of balancing effects such as have been described in my prior Patent No. 1,735,750 and obtain thereby relatively noise free and distortionless reception with alternating current operation of the receiver. Although different conditions of Operating a detector tube are Well known, the provision for quickly and easily Changing the detector circuit for best results as to sensitiveness or strong signal capacity as desired appears to be a desirable improvement.

If telephone as well as loudspeaker or other responsive device operation from a receiving set is desired, a plug-in jack S'l may be provided to receive a telephone connection plug. This jack S7 may be provided with a pair of Contacts SS which will be closed together when the telephone connection plug is inserted, and these Contacts may be Wired to the output circuit of tube T7 so that the loudspeaker or other responsive device is shorted When the telephones are used.

Simplified operation of the receiver described may be provided by a suitable grouping of controls and arranging the switches used to operate simultaneously. Since R7, R8 and condenser C5 are all Volume or antenna coupling controls, they may be operated on a single shaft as indicated in Figure 5 in which 10 is a shaft in Sections with insulated couplings 11 between them and S9 is a switch which short cirouits condenser C5 at one setting of the control shaft 10. As indicated in Figure 6, switches S5, SG and S10 may all be operated simultaneously and may be operated by means of the coils used in stage A3 or A4. Parts of switches SlO, 85, and SG are carried on an insulating member 12 capable of sliding or rotating a short distance and normally held in one position by a spring 13. A coil form is provided With a Stud or projection 14 which, when the coil form is inserted in its socket, makes contact With member 12 and changes its position to throw said switches as desired. Usually only one coil form in a set for different frequency ranges is provided With a switch Operating stud.

An alternative detector and audio amplifier circuit is shown in Figure '7, which also is designed in accordance with the features of my U. S. Patent No. 1,735,750. In the circuit of Figure 7, two power tubes Tl7, and T18 are shown with parallel input connections but separate output connections so that loudspeakers or other responsive devices having unlike characteristics may be operated advantageously. The output circuit of tube T17 includes choke L18 and blocking condenser C15 with terminals O O for connection to a responsive device. The output circuit of tube T18 includes choke L19, blocking condenser C16 and a primary 112 of a transformer having a secondary L13. Secondary L13 may be designed for connection to the moving coil of a dynamic type loudspeaker.

Power tubes T17 and T18 are operated in a circuit giving desirable balancing effects in combination with the operation of preceding tube TIG substantially as described in my patent referred to. Likewise, the input circuits of tube T16 and tubes T17 and T18 are made similar in nature so that performance, as affected by their input circuits, will be similar in nature with a resulting tendency of distortion introduced by tube TlG to compensate for distortion introduced by the power tubes.

Plate current for tubes T17 and T18 and tube T16 is supplied from the B plus connection, and it will be noted that signal currents in the plate circuit of tube T16 and tubes Tl'? and T18 are substantially opposed in direction. A point may be found in the plate circuit of tube T16, as between resistors R16 and R18, that may be approximately neutral with respect to signal currents handled by the amplifier and detector plate current may be supplied from this point to minimize feedback of signal currents. Feedback of signal currents may be further eliminated by a resistance and capacity filter comprising resistor R17 and condenser C1'7 or any equivalent. The output of detector tube T14 may be coupled to the input of tube 'I'l6 by means of any suitable coupling unit such as that represented by resistor R15, choke L15 and a direct current blocking condenser. Choke L15 may be made as a transformer or auto-transformer having a primary L14 which may be used for connection to a phonograph pickup or other device permitting the audio amplifier to be operated from some other source than detector Tlll. If T14 is a low impedance tube, its output may be coupled through primary L14 rather than L15 to obtain a Voltage step-up. If it is desired to operate tube T14 as a grid bias detector, a grid bias Voltage may be obtained from a tap on resistor R20 and filtered in a resistance and capacity filter comprising resistor R19 and the condenser C19, or any other known way of obtaining a grid bias Voltage may be employed.

In a resistance or impedance coupled amplifier, it is advantageous to have the cathodes of all tubes connected together. Grid impedance units such as L15 and L17 preferably should have considerable resistance to prevent resonance or tuning effects sufiicient to cause uneven frequency response. Grid bias resistors, such as R20, used without by-pass condensers, also may be effective in reducing resonance effects. The circuit of Figure '7 is designed to control the direction of feedback currents and tube Operating currents so that opposing or cancelling effects are obtained, and by selecting suitable tubes and circuit constants, useful balancing of an amplifier may be obtained with resulting stability and high quality performance.

A further improvement in audio amplifiers employing coupling, or direct current blocking, condensers such as 025, C26 and 028 in Figures 1 and '7 is the means shown for reducing the tendency of such condensers to become charged by grid current flowing when the grids of amplifier tubes to which they are connected swing on the positive halves of relatively strong signal waves. I have used resistors such as R25, R26 and R28 in series With the grids of tubes such as T5, TG and T16, respectively, as a means to reduce the tendency toward condenser charging and consequent distortion occurring in an amplifier. Said resistors may have a value from about 50,000 ohms to 500,000 ohms when used in common types of amplifiers, resistance values being selected in accordance With the values of circuit elements and the characteristics of the tubes employed. If a tube such as T5 has an input capacity of about 30 micro-micro-farads, there will be little loss of even such high audio frequencies as 8,000

reeassv or 10,000 cycles when R25 has a value of 250,000 ohms. R25 has a very different effect at radio frequencies; with the same tube and resistance value, the reduction of a radio frequency signal such as one megacycle will be very great. This effect of reducing radio frequency signals is desrable in an audio frequency amplifier, particularly after a detector tube. Although radio frequency chokes, R. F. C., and by-pass condensers are shown in Figures 1 and 7 in the detector output circuits, these chokes are not needed since resistors such as R25 and R28 may be used satisfacton'ly to keep radio frequency signals out of the audio amplifier.

Having thus described my invention, what I claim is:

1. A coupled electric circuit adapted for tuning selectively over a range of frequencies having input and output connections, an inductiveV reactance element connected acrosssaid input and having a value such that parallel resonance with capacity effects across the input is at a frequency substantially at the high frequency end of the selective tuning range of said coupled circuit, a capacitive reactance element and a coupling reactance element of such value connected in series with said inductive reactance element to make said series circuit resonant at a frequency lower than any within the selective tuning range of said coupled circuit, a selective tuning circuit including said coupling reactance element, and an output connection from said tuning circuit.

2. A coupled electric circuit adapted for tuning selectively over a range of frequencies having input and output connections, an inductive reactance element tapped intermediate of its ends With one part connected across said input and having such values in the two parts that parallel resonance with capacity effects across the input is at a frequency higher than any within the selective tuning range of said coupled circuit, a capacitive reactance element and a coupling reactance element of such value connected in series With said inductive reactance element to make said series circuit resonant at a frequency lower than any Within the selective tuning range of said coupled circuit, a selective tuning circuit including said coupling reactance element, and an output connection from said tuning circuit.

3. A coupled electric circuit adapted for tuning selectively over a range of frequencies having input and output connections, and an inductive reactance element connected across said input and having a value such that parallel resonance With capacity effects across the input is at a frequency higher than any within the selective tuning range of said coupled circuit, a capacitive reactance element and a coupling reactance element of such value connected in series with said inductive reactance element to make said series circuit resonant at a frequency lower than any Within the selective tuning range of said coupled circuit, a selective tuning circuit including said coupling reactance element, an output connection from said tuning circuit, a second coupled electric circuit having similar characteristics but some- What different resonant frequencies in the input and series circuit, and means for operatively coupling said two coupled circuits together.

4. A coupled electric circuit adapted for tuning selectively over a range of frequencies having input and output connections, an inductive reactance element connected across said input and having a value such that parallel resonance with capacity effects across the input is at a frequency higher than any within the selective tuning range of said coupled circuit, a coupling reactance element comprising an inductive part and a capacitive part, a capacitive reactance element of such value connected in series with said inductive reactance element and said coupling reactance element to make the series circuit so formed resonant at a frequency lower than any within the selective tuning range of said coupled circuit, a selective tuning circuit including said coupling reactance element, and an output connection from said tuning circuit.

5. A coupled electric circuit adapted for tuning selectively over a range of frequencies having input and output connections, an inductive reactance element connected across said input and having a value such that parallel resonance with capacity effects across the input is at a frequency higher than any Within the selective tuning range of said coupled circuit, a coupling reactance element comprising an inductive part and a capacitive part arranged in aiding phase and so selected in relative values that the coupling reactance varies directly with approximately the square root of frequency variation in the selective tuning range, a capacitive reactance element of such value connected in series with said inductive reactance element and said coupling reactance element to make the series circuit so formed resonant at a frequency lower than any within the selective tuning range of said coupled circuit, a selective tuning circuit including said coupling reactance element, and an output connection from said tuning circuit.

PHILIP H. GREELEY. 

